The present invention relates to a capacitor made of a metallized polypropylene film impregnated with a specified dielectric medium, which has a high withstanding voltage and a self-healing property, suitable for use under a high voltage and a higher potential gradient than ever, and more particularly relates to a capacitor comprising, as a dielectric material, metallized polypropylene film impregnated with a 1,2-bis(alkylphenyl)ethane represented by the general formula (I): ##STR1## wherein R.sup.1 represents a hydrogen atom or a methyl group; R.sup.2 represents a methyl group, ethyl group, propyl group or isopropyl group and n is 1 or 2, with the proviso that when R.sup.1 represents a methyl group, R.sup.2 represents a methyl group.
The conventional capacitors made of a metallized polypropylene film (polypropylene film on which a metal has been deposited by vacuum-evaporation) impregnated with a dielectric medium have been used under a relatively low voltage of lower than 600 V and a relatively low potential gradient, and are made by the process comprising the steps of winding a metallized polypropylene film (hereinafter referred to as MPP) to form an element, spraying a metalikon-metal onto the both ends of the thus formed element, thereby carrying out the metalikon (metal-spray-coating) thereon, welding lead wires to the portions carried out with metalikon, impregnating the thus formed element with an oil as a dielectric medium to obtain the oil-impregnated MPP capacitor.
The major reasons why such an oil-impregnated MPP capacitor could not sufficiently exhibit its expected characteristics lie in the occurrence of (1) the loosening of the bonding of the metalikon-metal and vacuum-evaporated and deposited metal, or in the extreme case, the separation of the two metals due to the deormation of polypropylene of MPP caused by the swelling thereof due to the oil impregnated into the element and (2) the separation of the metal particles which have been vacuum-evaporated and deposited onto polypropylene film, from the polypropylene film because of the weakening of the adhesion of the particles to the film. In the latter case, the electric characteristics of the oil-impregnated MPP capacitor such as corona potential, current-resistance, electric power loss and capacity are deteriorated, thereby the life of the capacitor is remarkably reduced.
On the other hand, in the case where a capacitor provided with foil-type electrodes is impregnated with an oil such as mineral oils of a low viscosity and good penetrance, alkylbenzene (hereinafter referred to as AB), alkylnaphthalene (hereinafter referred to as AN), 1-phenyl-1-xylylethane (hereinafter referred to 1,1-PXE), monoisopropyldiphenyl (hereinafter referred to as MIPB), etc. there are many problems such as (i) separation of particles of metalikon-metal from the film, (ii) reduction of adhesion of vacuum-evaporated and deposited metal to the particles of metalikon-metal, (iii) swelling of the part of the film at both ends of the element at the first stage of impregnation resulting in the clogging of the interlayer passage of the oil and thus resultant insufficient impregnation, (iv) reduction of the withstanding voltage due to the insufficiency of the oil in the part between the swelled layers of the film due to the absorption of the oil by polypropylene of MPP around the oil which has entered between the film layers after the completion of impregnation and, (v) the same reduction of the withstanding voltage due to the disturbed inter-layer circuration of the oil due to the raised viscosity of the oil caused by the dissolution of the film by the oil. These problems are caused by the large solubility of polypropylene (hereinafter referred to as PP) film into the oil and swelling of PP film by the oil.
In order to avoid the above-mentioned problems, as a better oil for impregnation into MPP condenser, a vegetable oil such as castor oil, an ester oil such as dioctyl phthalate, silicone oils, polybutene, waxes such as microcrystalline and low-molecular weight polyethylene, mixtures of these waxes and polybutene has been used. However, although the separation of sprayed metalikon-metal and vacuum-evaporated and deposited metal may be prevented by the use of the better oil, it is very difficult to have the element of MPP capacitor impregnated into innerpart thereof with such a highly viscous oil, and it is only effective to partially impregnate the adjacent parts of the electrode end. Accordingly, the MPP capacitor is not able to exhibit its characteristics with a restricted use thereof under a low voltage.
In addition, as a thin dielectric material, metallized insulating paper has been used, however, its dielectric loss is large due to the defect of paper itself and its withstanding voltage is also low. According to the defects of large dielectric loss, MPP has come to be used instead of metallized insulating paper. However, because of the difficulty in impregnating MPP capacitor with an oil, withstanding voltage has been scarcely improved.
Accordingly, the development of MPP capacitor of a high withstanding voltage has been strongly demanded.
The present inventors have studied for preparing MPP capacitor of a high withstanding voltage and as a result attained the present invention.